The present invention relates generally to torque assist motors for electric power steering systems and more specifically to a low-cost, injection-molded, buried, permanent magnet motor for use in an electric power steering system.
Permanent magnet brushless (PMBLDC or PMSM) motors may exhibit relatively high torque densities and are therefore useful in industrial drives for high performance applications. Permanent magnet (PM) motors with buried magnets are widely used in variable speed drives. Injection-molded buried permanent magnet (IBPM) motors typically employ materials injected into the rotor core and form rotor poles. This enhances the magnet retention and manufacturing yields for IBPM motors compared to surface-mount permanent magnet (SPM) motors. The retention of magnets is better in IBPM compared to the interior permanent magnet motor (IPM) with a rectangular bar magnet.
Magnet retention can be problematic in an interior permanent magnet motor (IPM) with a rectangular bar magnet, causing manufacturing yields for IBPM motors to suffer. Bar magnets also contribute appreciably to the cost of PM motors.
The placement of magnets inside the magnet pocket of an interior permanent magnet motor (IPM) with a rectangular bar magnet is an issue due to the manufacturing tolerance of both magnet bars and magnet pockets. This magnet placement creates certain ripple torque depending on the slot/pole combination of the motor. For high performance applications, torque ripple is an important challenge for PM motors (PMs) as it creates vibration and speed pulsation. Moreover, cogging torque minimization in IPM motor is more challenging compared to SPM motor as IPM allows smaller air gap and linear skewing or shaping of the magnet cannot be implemented in a feasible way due to the simpler rectangular shape of the PM motor.
Various techniques have been attempted to minimize the cogging torque but conventional techniques tend to add to the complexity and can negatively impact output torque. In addition, in motors employing sintered magnets, the increased complexity can contribute significantly to cost.
Magnet pole shaping, skewing of rotor magnets or stator structures, step-skewing of rotor magnets, combining slots and poles, magnet shaping, and incorporation of dummy notches in the stator teeth have been employed to minimize cogging torque in the PM motors. Unfortunately, however, these conventional techniques have caused other issues to arise. For example, the use of segmented stators, while bringing about improvements in slot fill and manufacturing time of the motor, have also given rise to certain undesirable harmonics, such as a large ninth order harmonic that has been experienced in one motor build and has been attributed to the gaps disposed between stator segments.
Accordingly, it is desirable to have an improved rotor design and techniques for imbedding magnets in rotors of IBPMs for high performance application such as Electric Power Steering. It is desirable to have systems and methods for overcoming issues relating to the ninth order harmonics that are inherent in conventional motors that employ a segmented stator.